The long- term objectives of this laboratory are to elucidate the immunopathologic basis of airway obstruction in a murine model of asthma. This proposal will investigate the role the epithelium plays in allergic lung inflammation and focus specifically on an epithelium- derived immunoregulatory molecule, Clara cell secretory protein (CCSP), that potentially limits deleterious allergic inflammation of the airway. Work from this laboratory has identified proteases derived from fungi and pollen as critical regulators of T helper activation and allergic inflammation. The dominant role played by proteases raises the intriguing possibility that the lung immune system has evolved mechanisms that limit tissue injury in the setting of allergic inflammation. Clara cells are non- ciliated respiratory epithelial cells that secrete one of the most abundant proteins contained in the respiratory lining fluid, CCSP. Although the biological functions of CCSP remain unclear, it is an antiprotease potentially capable of neutralizing exogenous proteases implicated in allergic airway disease. CCSP may also regulate adaptive immune responses of the airway by influencing epithelium-derived factors necessary for growth and survival of T and B cells. Studies from this laboratory provide preliminary evidence that CCSP down- regulates airway Th2 responses provoked by exogenous proteases. Thus, CCSP is immunosuppressive with regard to allergic lung inflammation but its precise mechanisms of action remain uncertain. Aim 1 of this proposal will define the allergic settings relevant to CCSP- dependent regulation. We will investigate whether the potency of both protease- containing and protease- deficient (ovalbumin/alum) allergens is inhibited by CCSP in vivo. The role of CCSP will be evaluated using mice overexpressing CCSP in the airway and CCSP-deficient mice. In part based on the results of Aim 1, we will dissect in Aim 2 the major mechanism by which CCSP attenuates allergic inflammation. We will focus specifically on T cell-dependent effects and explore, with and without CCSP, T helper effector differentiation in vitro and in vivo, Th2 homing to lung and Th2 activation and cytokine production in vitro. Where effects are observed, we will correlate immunosuppressive activities with the antiprotease potential of CCSP. Finally, we will explore the potential of CCSP in the prevention and amelioration of allergic lung disease. Mice will be challenged intranasally with recombinant CCSP during immune induction with protease- containing allergens and following established allergic lung inflammation to evaluate these two endpoints. Efficacy of CCSP will be compared to a specific protease inhibitor, streptomyces subtilisin inhibitor (SSI). Together, these data will elucidate an epithelium- dependent immunomodulatory mechanism of the lung mediated through CCSP, and potentially identify novel means for the therapy of allergic lung disorders such as asthma.